Figure 1: Gas turbine blade with cooling holes and flow passages.

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Introduction

Modern designs of jet engines have led to an increased pursuit to improve thermal efficiency. With inlet temperatures soaring over the melting point of earlier blade materials, superalloys have been developed and along with that, film-cooling is been incorporated to provide a cool insulating blanket.

In this article, we will be discussing how GridPro handles this broad class of problems since these holes, tubes, narrow passages are common geometrical features in engineering applications. Applications like ventilation duct holes, combustors, nuclear reactor fuel rods, etc have similar configurations to film cooling turbine holes. Since understanding the fluid flow pattern and their thermal behavior in these holes is critically important for the efficient, effective performance of these components, meshing them easily with high quality becomes important.

In Figure 2, some of the complex flow passages in turbine blades, gas turbines, and air-conditioning ventilation ducts are shown. In general, the meshing of holes/narrow tubes in connection with larger domains is easily possible with structured- multiblock tools. However, the complexity becomes overwhelming when the number of holes is large, especially when they are in hundreds if not in thousands as seen in the case of ventilation ducts.

In the case of film cooling holes in turbine blades, the challenge presents in the form of holes of varying cross-sections, inclinations, sizes, arrangement patterns, and placements. The simple-minded approach/or a scriptable approach of creating blocks for one hole and copy translating them to position for the rest doesn’t hold good anymore.

Conventional topology building systems will demand block creation manually for each hole. It is a no-brainer to see why such geometries are considered beyond the scope of structured grid-generators. Hitherto, hybrid unstructured and cartesian gridding methods have been the meshing approaches in the industry for such configurations.

GridPro in the Blade Trenches

To address the gridding challenges, in GridPro we have developed a tool labeled as hole-topology. The algorithm constructs the blocks for multiple holes, interconnects them effortlessly. What would have taken days to create blocks for such multi-hole scenarios gets reduced to a few minutes. The tool does most part of the heavy lifting and engineers can take the output of the blocks created and extrude to fill the rest of the domain.

Some scenarios which can be handled currently by the tool are (1) Holes with different cross-sections from circle to ellipsoid to super-ellipsoid. (2) Varying lengths and inclinations. (3) Different meshing zones. (4) Varying hole patterns.

Varying Cross-Sections:

The blocking pattern identifies the shape of the holes and creates an exclusive grid based on the shape. Figure 4, shows grid images for a few of these geometric variants.

The algorithm easily responds to the changes in cross-section and shapes. This will be a very handy tool in the early stages of the design optimization cycles, where the designer will be experimenting with various sections and sizes to figure out what suits best. One of our partners, Friendship Systems in a recent case study show the optimization of Turbine blade cooling holes.

The animated videos 1-2, show the grids adapting to the different shapes and cross-sectional sizes in a typical turbine blade cooling holes design cycle. If the hole shapes and sizes need to be studied, the blocking built for one pattern can be used as a template for generating grids for various shapes.

Varying Lengths and Inclinations:

The holes of varying lengths and inclinations can be blocked appropriately by the algorithm. The user can exercise control to input a varying number of blocks depending on the length of the tubes. It not only takes care of the hole shapes but also various inclination angles. Some of the meshes seen here are holes that incline to an angle of 7 degrees. Figure 5, shows meshes created for tubes typically seen in gas turbine blades.

A noteworthy aspect is that, even for highly inclined holes, the grid quality is not compromised. Figure 6, shows sectional views of the grids in and around the holes.

Varying Meshing Zones:

The tool provides flexibility to choose meshing zones, depending on the requirements, and accordingly, the blocking patterns are changed. Figure 7, shows 3 possible scenarios. The first scenario has mesh inside the plenum, hole, and surface of the blade. In the second scenario, if the user prefers not to mesh the plenum the tool automatically builds an inlet boundary at the outer surface of the plenum so that the pipe section and the outside of the blade only get meshed. And in the third scenario, if the user chooses to only mesh the outside of the blade but wants the hole outlet boundaries to be marked, the mesh can be created only on the surface of the blade with holes marked with boundary conditions.

Varying Hole Patterns:

Hole pattern arrangements like linear, rectangular, and circular are automatically recognized and the inter-linking of blocks is established. Figure 8, shows the pattern recognition and block generation steps for a rectangular pattern.

A CFD Mesher’s Treatise

A combination of automatic hole capturing and automatic coarsening of mesh is a feature that every mesher would love to have. Here we display that in a scenario like the ventilation duct in a room. This is a very powerful combo for cases where the geometric scales are large. The holes have dimensions in millimeters while the room dimensions are in meters.

Conventional blocking strategy would have culminated in generating a grid with a massive cell count and CFD engineers would have rejected it as being computationally expensive.

The automatic blocking of holes paves the way for the Nesting tools like the clamp-nesting and reverse nesting to systematically reduce the size of the blocks by looping back to create larger blocks. Figure 9-11, shows snaps shots of the grid generated using this unique strategy.

In fact, this combo strategy is so effective it beats the unstructured grid-generator in cell-count hands-down. For the above case of ventilator duct in a room, the unstructured approach needs 36 million to discretize the domain while hole-topo/nesting combo generates a much finely discretized grid for 16 million!!!

Conclusion

The benefits of this tool are immense. Not only does it reduce the blocking time for multi-holes geometries, but also expands the wide variety of geometric complexity structured-blocking can be extended to. What was previously considered as the purview of unstructured grids is now eclipsed by the hole-topology structured blocking approach. Not only does it brings the well-known benefits of flow-alignment, faster solver convergence, higher accuracy, but also reduces the cell count drastically and makes the whole CFD computations cheaper and affordable even for such class of complex problems.

Further Reading

1. Art and Science of Meshing Turbine Blades
2. Nesting your way to mesh Multi-Scale CFD Simulation!

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